Homonuclear molecules, or homonuclear species, are molecules composed of only one type of element. Homonuclear molecules may consist of various numbers of atoms, depending on the element's properties. Some elements form molecules of more than one size. Noble gases rarely form bonds, so they only have one atom. The most familiar homonuclear molecules are diatomic, meaning they consist of two atoms, though not all diatomic molecules are homonuclear. Homonuclear diatomic molecules include hydrogen (H2), oxygen (O2), nitrogen (N2) and all of the halogens. Oxygen, in addition to its diatomic form, also forms the homonuclear triatomic molecule, ozone (O3). Homonuclear tetratomic molecules include arsenic (As4) and phosphorus (P4). Sulfur forms diatomic, hexatomic and octatomic homonuclear molecules (S2, S6, S8), though the first two are rare.